1. Field of the Invention
The invention is generally related to a capacitive readout circuit of a touch panel, and more particularly, to a switched-capacitor tracking apparatus of the touch panel.
2. Description of Related Art
In many electronic products, a capacitor characteristic (i.e. capacitance) of a target component is read out frequently. For instance, variations in the capacitances of the currently popular touch panels are detected by the readout circuit. Generally speaking, the touch panel can be classified into a resistive touch panel, an optical touch panel, and a capacitive touch panel, etc. Compared with a keyboard and a mouse, the touch panel is a simpler, more direct input/output interface. Therefore, the touch panel is usually applied as a man-machine interface between a man and an electronic device. The capacitive touch panels are operated by approaching or touching the touch panels with a finger or a conductive material to change the capacitances of the touch panels. By reading the circuitry to detect the capacitance variations of the capacitive touch panels, the touch locations of the conductive body or the finger on the touch panel can be detected.
FIG. 1 is a schematic block diagram illustrating a function of a conventional touch panel. A touch panel 14 includes a plurality of pixel capacitors. FIG. 1 illustrates a panel capacitor (i.e. a target capacitor 16) as a representative example. The data from the target capacitor 16 is transmitted to the analog to digital converter 18 (ADC 18). Consequently, the ADC 18 can convert a capacitance of the target capacitor 16 to a digital value. The ADC 18 illustrated in FIG. 1 consumes great effort for changing whole capacitance of the target capacitor 16 to the digital value. If the capacitance range of the target capacitor 16 is from 0 pF to 50 pF, then an output of the ADC 18 is 16 bits. When the capacitance of the target capacitor 16 varies from 25 pF to 25.1 pF (wherein 25 pF is DC component of the capacitance, 0.1 pF is AC component of the capacitance), a digital code outputted by the ADC 18 changes from 32768 (i.e. 216×25÷50) to 32899 (i.e. 216×25.1÷50). The ADC 18 has always needed to converts the DC component of the capacitance of the target capacitor 16.